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Wednesday, March 23, 2011

Current levels of Iodine 131 in Tokyo tap water converted to micro sieverts per hour

At present, there is a warning in Tokyo for infants and very young children to refrain from consuming tap water. It might be helpful to understand how this translates to actual exposures per unit of water consumed.

So the question is: How much radiation exposure would a person get if they consumed 2 liters of tap water a day in Tokyo?

Tokyo is reporting 210 becquerels of radioactivity found in 1 kilogram of tap water.

1 kilogram ~ 1 liter of water

If consumed and present in the body, 1 liter of water containing 210 becquerels of iodine 131 activity will give off an internal dose of 0.0968404143996599 microsievert/hour (after each photon ejected from the iodine nucleus travels a depth of 1cm in tissue and deposits its dose).

If a person consumes 2 liters of water a day, at any given time the internal dose in body tissue will yield about 4.6483 microsieverts over a 24 hour period. Assuming a constant contamination of the environment, and subsequently the drinking water, 30 days of drinking 2 liters of tap water a day would yield an internal exposure of 13.94 millirem. One year of consumption at these levels would come out to about 167 millirem.

These levels are quite tolerable for adults if exposures are short term, and even if you add current background readings in air from Tokyo, adults would still find themselves below the MPD for the general population (500mr/yr). Because water is consumed and distributed throughout body tissue, and because children are more susceptible to cancer, the authorities in Japan were wise to be precautionary. Most of iodine would target the thyroid.

If the radioactive venting from Fukushima were to end today, Tokyo tap water would be safe to drink in less than a weeks time (assuming negligible amounts of Cesium).

The activity at Fukushima, however, continues to make the situation unpredictable.

Feel free to peer review

Update: Smoke billowing from reactor 2, Radiation Level At Fukushima Reactor No. 2 At Its Highest Level Recorded So Far

50 Rem/hr! Looks like they will have to retreat until that dose pares back.